preservation of mice

A question for the group.
I'm planning an experiment in which mice will be killed and the
lungs and several other organs examined histologically. Some of
the mice will be transgenic. The question is, would killing the
animals with CO2 narcosis have an adverse impact on the quality
of preservation, e.g. generating formalin pigment in the spleen?
Does anyone have experience with this method?
Thanks in advance
Hal Hawkins
UT Medical Branch, Galveston, TX